Four Passes

gistfile1.js

for(;i<nums.length;i++) {
   var northeast = nums[i]*nums[i-(n-1)]*nums[i-2*(n-1)]*nums[i-3*(n-1)],
   if(northeast > p) p = northeast;
}
for(;i<nums.length;i++) {
   var north = nums[i]*nums[i-n]*nums[i-2*n]*nums[i-3*n];
   if(north > p) p = north;
}
for(;i<nums.length;i++) {
   var east = nums[i]*nums[i+1]*nums[i+2]*nums[i+3];
   if(east > p) p = east;
}
for(;i<nums.length;i++) { 
   var northeast = nums[i]*nums[i+(n+1)]*nums[i+2*(n+1)]*nums[i+3*(n+1)];
   if(southeast > p) p = southeast;
}